Rich, You wrote 'you wrote 'just look at all the WRONG theories of how a wing/sail works and these incorrect 'hypotheses' are still offered in 'sailing' books and highschool 'science' courses ... and just about everyone accepts them as fact when in fact they're dead wrong'' in a thread on fuel. What do you mean by this? thanks
RichH - Sail 'Theory'
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Arvel Gentry is the one who debunked most of this
popular but still incorrect theory. The Wright Brothers would entirely disagree with whats being offered today in sailing books, etc. In the early 1970s an aerodynamicist/sailor technically debunked the 'slot effect', etc. and offered many technical articles based on straight aerodynamics that essentially set the sailing world on its collective ear. Even the then leading 'authorities' of sailing theory (C.J.Marchaj, etc.) then deferred to these works/articles by A. Gentry as being the 'solution' to how a sail actually gives lift. These articles can be found at www.arvelgentry.com in the 'magazine articles' and 'technical articles'. Although they were written for the 'common man' they are based within aerodynamics and are thus quite difficult for the average person to understand ..... you have to read them over and over several times so that they make sense; as, aerodynamics is definitely NOT an intuitive science. The 'key' article was "How_Sails_Really_Work, April 1973" and is found in the magazine section of that website. Gentry became the 'secret weapon' for the success of the US effort of the Americas Cup all through the 70s and into the 80s. His articles of the use of tell-tales are probably the *most important* for the common sailor: Checking Trim on the Wind, November 1973 Achieving Proper Balance, December 1973 Sailing to Windward, January 1974 Are You at Optimum Trim?, March 1974 .... all found in the 'magazine articles' section hope this helps.
popular but still incorrect theory. The Wright Brothers would entirely disagree with whats being offered today in sailing books, etc. In the early 1970s an aerodynamicist/sailor technically debunked the 'slot effect', etc. and offered many technical articles based on straight aerodynamics that essentially set the sailing world on its collective ear. Even the then leading 'authorities' of sailing theory (C.J.Marchaj, etc.) then deferred to these works/articles by A. Gentry as being the 'solution' to how a sail actually gives lift. These articles can be found at www.arvelgentry.com in the 'magazine articles' and 'technical articles'. Although they were written for the 'common man' they are based within aerodynamics and are thus quite difficult for the average person to understand ..... you have to read them over and over several times so that they make sense; as, aerodynamics is definitely NOT an intuitive science. The 'key' article was "How_Sails_Really_Work, April 1973" and is found in the magazine section of that website. Gentry became the 'secret weapon' for the success of the US effort of the Americas Cup all through the 70s and into the 80s. His articles of the use of tell-tales are probably the *most important* for the common sailor: Checking Trim on the Wind, November 1973 Achieving Proper Balance, December 1973 Sailing to Windward, January 1974 Are You at Optimum Trim?, March 1974 .... all found in the 'magazine articles' section hope this helps.
Rich...
Check out Wallace Ross's "Sail Power", which doesn't necessairly support the "slot effect" as much as "flow attachment" to the sails themselves, which is why Gentry's tell-tales are so significant.
Check out Wallace Ross's "Sail Power", which doesn't necessairly support the "slot effect" as much as "flow attachment" to the sails themselves, which is why Gentry's tell-tales are so significant.
I'll give it a shot...
Prior to Genty's articles, everyone believed that the airflow coming onto the slot between the main and jib was channelled and accellerated, causing a low pressure area to form due to the Bernoulli effect. The low pressure on the lee side of the main was believed to pull the boat along to windward, and the function of the jib was mainly to channel the airflow. All new sailors were taught this, and all sailing books illustrated it with pretty drawings. Gentry showed, on the other hand, with wind tunnel tests, that the incoming airflow actually spread out to avoid the slot. The sails worked in tandem, the jib helping to keep airflow attached to the lee side of the main, and the main deflecting airflow at its leading edge so that the jib enjoyed a permanent 'lift'. Aerodynamically, upwind sailing might better be understood as the sails exerting a force on the air to change its direction of flow, and the air exerting an equal and opposite force on the sails, in such a direction that the boat is partly heeled over and partly driven forward. That's what I got out of it, anyway. Anybody else?
Prior to Genty's articles, everyone believed that the airflow coming onto the slot between the main and jib was channelled and accellerated, causing a low pressure area to form due to the Bernoulli effect. The low pressure on the lee side of the main was believed to pull the boat along to windward, and the function of the jib was mainly to channel the airflow. All new sailors were taught this, and all sailing books illustrated it with pretty drawings. Gentry showed, on the other hand, with wind tunnel tests, that the incoming airflow actually spread out to avoid the slot. The sails worked in tandem, the jib helping to keep airflow attached to the lee side of the main, and the main deflecting airflow at its leading edge so that the jib enjoyed a permanent 'lift'. Aerodynamically, upwind sailing might better be understood as the sails exerting a force on the air to change its direction of flow, and the air exerting an equal and opposite force on the sails, in such a direction that the boat is partly heeled over and partly driven forward. That's what I got out of it, anyway. Anybody else?
sounds a bit the same
Maybe Gentry went too far, but channeling the airflow through the slot and helping keep the airflow attached to the lee side of the main sound a lot alike. I definitely know that the jib does alter the flow of the air through the slot. When I'm sailing on a 40 degree awa with main alone, I have to let the boom out a lot more then when I have the jib up too. Of course, one would be pretty foolish to think that the jib didn't create lift.
Maybe Gentry went too far, but channeling the airflow through the slot and helping keep the airflow attached to the lee side of the main sound a lot alike. I definitely know that the jib does alter the flow of the air through the slot. When I'm sailing on a 40 degree awa with main alone, I have to let the boom out a lot more then when I have the jib up too. Of course, one would be pretty foolish to think that the jib didn't create lift.
The real difference
is that before, the airflow was thought to be concentrated or compressed and accellerated into the slot, but Gentry showed that the airflow shunned, or spread out to get away from the slot. As I understand it.
is that before, the airflow was thought to be concentrated or compressed and accellerated into the slot, but Gentry showed that the airflow shunned, or spread out to get away from the slot. As I understand it.
Sandy is close .....
Sandy is essentially correct... as the air essentially moves out of the way (upwashes) to go around the saiL(s). The Mainsail makes the jib/genoa more efficient by 'upwash' and the jib makes the main more efficient by 'bootstrap' effect. .... and the biggy is that air ****re-circulates**** around the whole set of sails and the LIFT is due principally to a complicated effect because the air has viscosity, etc. .... all *non-intuitive* but can be 'seen' if one properly uses a 'full-set' of tell-tales, etc.
Sandy is essentially correct... as the air essentially moves out of the way (upwashes) to go around the saiL(s). The Mainsail makes the jib/genoa more efficient by 'upwash' and the jib makes the main more efficient by 'bootstrap' effect. .... and the biggy is that air ****re-circulates**** around the whole set of sails and the LIFT is due principally to a complicated effect because the air has viscosity, etc. .... all *non-intuitive* but can be 'seen' if one properly uses a 'full-set' of tell-tales, etc.
So sails are like a two blade trubine?
This same effect happens in a gas turbine compressor section. The angle of attack of the blades are all the same but they are not the angle that a single blade would have to have to not "stall" Each blade helps the one next to it to move the air more efficiently. Another way to look at it is the sails are like two rocks in a stream. The air does not want to go through the small slot and most of it goes around or over the whole boat. This change in flow is cause by a pressure force from the sails which have an equal and opposite force on the that heel and drive the boat. I went to the science museum in Baltimore where they have a water flume for the kids to play with making dams and such. Well one thing led to another and it was quite enlightening what the forces are on the blocks when you angle them like a jib and main. Course water is not air but they are both fluids
This same effect happens in a gas turbine compressor section. The angle of attack of the blades are all the same but they are not the angle that a single blade would have to have to not "stall" Each blade helps the one next to it to move the air more efficiently. Another way to look at it is the sails are like two rocks in a stream. The air does not want to go through the small slot and most of it goes around or over the whole boat. This change in flow is cause by a pressure force from the sails which have an equal and opposite force on the that heel and drive the boat. I went to the science museum in Baltimore where they have a water flume for the kids to play with making dams and such. Well one thing led to another and it was quite enlightening what the forces are on the blocks when you angle them like a jib and main. Course water is not air but they are both fluids
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